Recently, liquid crystal displays using a-Si:H (hydrogenated amorphous silicon) TFTs or poly-Si (polycrystalline silicon) TFTs are provided with an automatic back-light adjusting function or a touch screen function utilizing light sensors. In this type of liquid crystal displays, light sensor elements are configured with the same structure as that of thin film transistors (TFTs) provided as switching elements for pixels (refer to, for example, Japanese Patent Laid-open No. 2007-018458). Therefore, it is possible to inexpensively provide a display equipped with light sensors, without spoiling such advantageous features as reduced size and reduced thickness.
Hitherto, a layer contributing to photoelectric conversion by sensing light in the light sensor element (the layer will hereinafter be referred to as “photoelectric conversion layer”) has been formed in the same step as that for forming a channel layer in the thin film transistor serving as the switching element for the pixel. Therefore, the photoelectric conversion layer in the light sensor elements and the channel layer in the thin film transistors are formed in the same thickness over a substrate.
However, in the liquid crystal displays using the a-Si:H TFTs or poly-Si TFTs, generally, the channel layer is composed of a very thin film in order to maintain good transistor characteristics. In this case, the photoelectric conversion layer is composed of a very thin film, like the channel layer. In the displays equipped with light sensors according to the related art, therefore, most of the light externally incident on the light sensor element would be transmitted through the photoelectric conversion layer, making it very difficult to obtain a sufficient sensor sensitivity.
Besides, the channel layer in a poly-Si TFTs is generally formed in a thickness of 50 nm to 100 nm. If the photoelectric conversion layer is formed in a film thickness similar to that of the channel layer, for example, a thickness of around 50 nm, visible light is mostly transmitted through the film portion of the photoelectric conversion layer, irrespectively of whether the film portion is formed of poly-Si or a-Si. The light thus transmitted does not contribute to generation of electron-hole pairs and, therefore, the sensitivity of the light sensor element is lowered.
FIG. 35 is a graph plotted with light wavelength (λ) taken on the axis of abscissas, absorption coefficient (α) taken on the left axis of ordinates, and film thickness at which light intensity becomes 1/e taken on the right axis of ordinates, in the case where poly-Si is used to form the channel layer and the photoelectric conversion layer. Similarly, FIG. 36 is a graph plotted with light wavelength (λ) taken on the axis of abscissas, absorption coefficient (α) taken on the left axis of ordinates, and film thickness at which light intensity becomes 1/e taken on the right axis of ordinates, in the case where a-Si:H is used to form the channel layer and the photoelectric conversion layer.
As is seen from FIGS. 35 and 36, a film thickness of at least 100 nm and above may be needed to ensure good light absorption. To enhance the sensitivity of the light sensor, it may be contemplated to increase the film thickness at the parts corresponding to the channel layer and the photoelectric conversion layer. In the case of the poly-Si TFT, however, an increase in the film thickness leads to problems such as a rise in the OFF current of transistor, an increase in light leakage, and a difficulty in crystallization by a laser annealing treatment using an excimer laser. In the case of the a-Si:H TFT, also, an increase in the film thickness leads to problems such as a rise in the OFF current, an increase in S-D resistance, and an increase in light leakage.
The present invention has been made in order to solve the above-mentioned problems. Accordingly, it is an object of the present invention to provide a display, and a method for manufacturing the same, by which it is possible, in the case of forming switching elements and light sensor elements over an underlying layer of a substrate, to enhance the sensitivity of the light sensor elements, without affecting the characteristics of the switching elements, by controlling the sensitivity characteristic of the light sensor elements separately from the switching elements.